Aero-derivative gas turbine engines are hybrid gas turbine engines that typically take a compressor, a combustion system, and a turbine from a gas turbine engine used in an aeronautical application, such as powering an aircraft, and combine it with a power turbine to drive an electrical generator. Typically the turbine in an aero engine is designed to drive only the compressor, and it exhausts combustion gasses at a pressure sufficiently high to propel an aircraft due to the pressure difference between the engine exhaust and ambient air. For aero-derivative applications, a power turbine is added to the turbine of the aero engine to harness extra energy from the combustion gasses prior to exhaust in order to drive an electrical generator for the generation of electrical power.
The aero engine, the power turbine, and the generator may be connected in various configurations. In a first configuration the power turbine is not connected to the aero engine, but simply receives exhaust from the aero engine and that exhaust provides the energy to rotate the power turbine. In this configuration a power turbine shaft is an additional shaft and it connects the power turbine to the generator. In a second configuration the aero engine and the power turbine may be rotationally connected. The connection may be accomplished by replacing a low pressure shaft of the aero engine with an aero-derivative low pressure shaft. The aero-derivative low pressure shaft may extend further aft through the power turbine and be supported by an additional bearing set aft of the power turbine. In order to connect the generator, the aero-derivative shaft may extend further aft to a downstream disposed generator. Alternately, an upstream end of the shaft may extend further upstream to an upstream disposed generator. In both variations of the second configuration the power turbine shaft is the same as the low pressure shaft.
Aero-derivative engines are often used to provide electrical power for small installations or to provide supplemental power for generation facilities during periods of increased demand. They may be used to supplement industrial gas turbine engine electricity production, or they may be standalone. In a standalone configuration it is common to assemble two aero-derivative engines to one generator for power generation. One such configuration would entail one aero-derivative engine configured to rotate in a first direction (for example, having right hand blades) and have a power turbine shaft extend aft to connect to a first end of a generator shaft. A second aero-derivative engine configured to rotate in the opposite direction (for example, having left hand blades) may have a power turbine shaft extend aft to connect to a second end of the same generator shaft. In this configuration both aero-derivative engines are exhausting toward each other, but are rotation in the same direction with respect to the generator. The result is that the power turbine shaft of the first aero-derivative engine is connected to a generator through-shaft, which is also connected to the power turbine shaft of the second aero-derivative engine. Whether or not the respective power turbine shaft is connected to the low pressure shaft of the respective engine depends on the configuration of the respective gas turbine engine as described earlier. Aero gas turbine engines are smaller, lighter, and portable, whereas industrial gas turbine engines typically used to generate power are larger, heavier, and not portable. However, in order to be smaller and lighter, aero-derivative gas turbine engines are typically made of more expensive, less durable materials. Further, combustion in annular combustors typically used in aero engines is purposefully robust to ensure a reliable flame under variable engine demand conditions, but as a result, aero gas turbine engines commonly generate greater levels of pollutants such as NOx than their industrial counterparts. Additionally, maintenance of aero-derivative engines may be more frequent and difficult than for industrial gas turbine engines. Consequently, there remains room for improvement in the art.